This invention relates to electro optic modulators.
It is known to form p-i-n devices around ridge waveguide structures. A summary is shown in FIG. 1, where the ridge 10 is surrounded by a slab region 12, 14 on either side, together forming a single mode ridge waveguide. The device is protected with an oxide layer 16, in which openings 18, 20 are formed either side of the ridge 10. These regions are doped to form local p and n regions, thus creating a p-i-n diode across the waveguide. This can be used to manipulate the charge carrier concentration within the waveguide, thus controlling its refractive index. This can then be used to modulate a light beam passing through the waveguide. Alternatively, a n-i-n or p-i-p structure will be sufficiently conductive to provide a thermal phase modulator by controlling the refractive index through its temperature dependence. Such a structure is described in U.S. Pat. No. 5,757,986, which this application is a CIP application thereof and whose entire disclosure is incorporated herein by reference.
As illustrated, the device is a silicon-on-insulator structure in which the active components are formed over an insulating oxide layer 22 on a supporting silicon substrate 24.
The dopant in this arrangement is controlled to remain in the substrate region on either side of the waveguide so that the single mode light beam does not meet a doped region. This is because dopant elements tend to act as absorption sites for the light beam, leading to signal losses and local heating.
Dopant is added to such structures in a generally well-known manner, in which the surface of the silicon is exposed to a dopant containing gas. Areas which are not to be doped are covered with a protective layer of SiO2. Dopant then travels through the silicon according to the diffusion equations, which stipulate that for a constant concentration of dopant at the surface, the dopant concentration within the silicon will decline exponentially with distance, but that, as time progresses, the rate of that decline with distance will fall. Effectively, this means that the concentration at any one point will increase with time.
A difficulty in manufacturing the structure shown in FIG. 1 is that the dopant profile is difficult to control, as a result of the above. It tends to adopt the shape 26 shown in FIG. 1, in which sideways diffusion takes place at the edges of the opening in the protective SiO2 layer. The shape which results means that the current density and current path will vary across the vertical extent of the light mode. Ideally, the current density would be controlled to maximise the overlap between the electrical current and the optical mode.
It is possible to heat treat a doped region such as to drive the dopant into the slab region. A deeper dopant profile such as this might create would be preferable per se. However, this process will tend also to spread the dopant horizontally, with the result that the doped areas would then need to be more widely separated from the waveguide. This in turn is undesirable. The wider doped areas would also lead to a less preferred current density profile.
The present invention seeks to provide an electro optic device in which the dopant profile allows a more appropriate current density profile to be established during use.
In its first aspect, the present invention provides an electro-optic device comprising a ridge waveguide surrounded on either side by a slab region containing a doped region, thereby to form a conductive path across the waveguide, the doped region being bounded on at least two sides by a confining layer of a material different to the material of the slab region.
In its second aspect, the present invention provides an electro optic device comprising a ridge waveguide surrounded on either side by a slab region containing a doped region, the slab region defining a plane, thereby forming a conductive path across the waveguide, the doped region having a substantially uniform distribution in a direction perpendicular to said plane.
In a third aspect, the invention provides an electro optic device comprising a ridge waveguide surrounded on either side by a slab region containing dopant, the slab region defining a plane, thereby to form a conductive path across the waveguide, the dopant having been diffused into the slab region in a direction substantially perpendicular to said plane.
In a fourth aspect, the invention provides an electro optic device comprising a ridge waveguide surrounded on either side by a slab region containing dopant, thereby to form a conductive path across the waveguide, the dopant having been diffused into the slab region from a side surface of an etched region formed in the slab region.
The invention also relates to a method of forming an electro optic device comprising the steps of forming a ridge waveguide on a slab surface the slab surface defining a plane, etching a region of the slab surface on at least one side of the waveguide and applying a dopant to a side surface of the etched region thereby to introduce the dopant into the substrate in a direction substantially parallel to said plane.
It is possible to further etch the etched region with an anisotropic wet etchant. This will leave the etched region with internally relieved sides, a profile which will be transferred to the dopant profile. This can be used to tailor the conductive region, for example to provide a peak current density co-incident with the peak light intensity of the mode distribution.
Other preferred features of the invention will be apparent from the following description and the subsidiary claims of the specification.